The present invention relates in general to a combined temperature and liquid level sensor and control and pertains, more particularly, to a combined sensor for providing a dual control signal directed to form a single unit that may include a desired, inherent time delay to prevent rapid relay cycling when a liquid moves. The parent application, Ser. No. 07/490,472, was allowed on Mar. 12, 1991, and issued Oct. 15, 1991 as U.S. Pat. No. 5,056,363.
With the conventional sensor and control devices for temperature control, different kinds of sensors may be enclosed in a housing well and immersed in a liquid whose temperature is being controlled. The conventional liquid level sensors and controls use electrical probes immersed in a liquid which conducts an electrical current.
These conventional sensors and controls are disclosed in a number of patents. The Ohtani patent, U.S. Pat. No. 4,825,695, reveals a liquid level sensor mounted on a frame which restricts the movement of the sensor. The Gerve patent, U.S. Pat. No. 4,777,821, reveals a sonic sensor for detecting a liquid level within a measuring chamber and includes a gas thermometer for detecting the temperature of a liquid in a measuring chamber. This liquid provides a reservoir for measuring a temperature difference.
The Chandler patent, U.S. Pat. No. 4,660,026, reveals a fluid state detector and includes two thermistors. One thermistor is exposed to air and another thermistor detects a liquid level. A resistance change is detected when the thermistor is immersed within a liquid. The Marthland patent, U.S. Pat. No. 4,630,477, reveals a thermistor encapsulated within a corrosive resistant material. A change in resistance provides a measurement of a level of a corrosive liquid.
Both the Puscanser, U.S. Pat. No. 4,638,291, and the Blatnik, U.S. Pat. No. 4,277,773, patents reveal liquid level sensors utilizing electrical probes. Detection of a liquid level occurs when a conductive liquid immerses both probes, thereby providing a path for current to flow.
The Minorigaya patent, U.S. Pat. No. 4,135,186, reveals a combination liquid level and high temperature detecting apparatus including a thermistor in combination with a relay coil inside a casing. A variation in current value at the thermistor caused by liquid temperature variation is corrected by an associated relay and resistance variation. The liquid level apparatus is intended to provide accurate readings independent of liquid temperature variation and an excess temperature sensor.
Another patent, U.K Patent No. 2,022,844A, reveals a liquid level and temperature probe combination including a pair of leads and one thermistor providing liquid level sensing as a function of heat dissipation and another thermistor providing conventional thermistor temperature sensing, also as a function of heat dissipation.
Among the drawbacks of conventional devices is the need for two separate probes, one for temperature detection and one for liquid level detection. A single unit or probe house both the temperature sensing and liquid level sensing devices in the present invention. The temperature sensor can be a thermistor or other comparable devices, e.g., a thermocouple.
The level sensing means of a conventional device may utilize electrical contacts to sense a change in conductivity or resistance due to immersion within a conductive liquid. In conventional devices, it is also true that liquid splashing or rapid movement of the liquid may result in an incorrect liquid level signal.
Conventional devices include other drawbacks, including those resulting from a lack of adjustment means needed as a change in fluid electrical resistance occurs over time as contaminants build up in the fluid. Conventional liquid level sensors using alternating current sensors operating at standard frequencies of 50 or 60 HZ can cause rapid corrosion and corrosive build up on an exposed tip of a sensor housing or tube.
Accordingly, it is the object of the present invention to provide an improved integral liquid level and temperature sensor for detecting and providing suitable output signals for controlling the level and temperature of a liquid.
Another objective of the present invention is to provide an improved integral liquid level and temperature sensor that is economical and provides a single unit housing sensors for both level and temperature conditions.
A further objective of the present invention is to provide an improved integral liquid level and temperature sensor by combining the sensors for level and temperature. The sensor of this invention eliminates the need to penetrate a liquid vessel more than once for placement of sensors. This feature should result in an overall lower installation cost when compared to conventional devices.
Still another object of the present invention is to provide an improved integral liquid level and temperature sensor which reduces the effects of corrosion on the sensor body.
Still a further object of the present invention is to provide an improved integral liquid level and temperature sensor which incorporates an inherent time delay in a control circuit. The time delay acts to prevent rapid cycling of circuit control relays due to liquid movement.
Another object of the present invention is to provide an improved integral liquid level and temperature sensor which includes an inherent adjustment for different fluid resistivities. Thus, different sensors are not automatically required for use with different liquids.
A further object of the present invention is to provide an improved integral liquid level and temperature sensor that controls a heater as a result of two variables, i.e., both temperature and liquid level. Thus, catastrophic failure of over temperature and heater burn out may be prevented. The arrangement of the present invention provides an economic control not requiring two separate, independent controls.
Still another object of the present invention is to provide an improved integral liquid level and temperature sensor that operates with a relatively high frequency, e.g., 10K HZ or more. The high frequency, alternating current used to detect the liquid level substantially reduces the electrolysis effect, thereby reducing housing corrosion at the closed end exposed to the conductive liquid. Periodic cleaning or replacement of the sensor is significantly reduced.